"Many models have suggested the heliotail might look like this or
like that, but we have had no observations," David McComas, IBEX
principal investigator at Southwest Research Institute in San
Antonio, Tex., said in a statement. "We always drew pictures
where the tail of the solar system just trailed off the page,
since we couldn't even speculate about what it really looked
like." [ Images:
NASA's IBEX Sees Our Solar System's Tail ]

The tail drags behind the bullet-shaped heliosphere, or the
bubble surrounding our solar system that's created by the solar
wind and solar magnetic field. Our heliotail is "a much larger
structure with a much more interesting configuration" than
scientists had previously predicted, McComas added during a news
conference announcing the finding.

Researchers say the comet-like tail is inflated by the solar wind
of particles streaming off the sun, and the four-leaf clover
shape is the result of fast solar wind shooting out near the
sun's poles and slower wind flowing from near the sun's equator.
The finding is based on the first three years of IBEX's
measurements of energetic neutral atoms.

In the interstellar boundary region, charged particles from the
sun stream outward far beyond the planets toward the gas- and
dust-filled space between stars. Collisions between these
particles and interstellar material create fast-moving particles
with no charge, known as energetic neutral atoms, or ENAs. Some
of these particles speed inward toward the sun, where IBEX can
detect them from its perch 200,000 miles (322,000 kilometers)
above Earth.

"Scientists have always presumed that the heliosphere had a
tail," Eric Christian, IBEX mission scientist at Goddard Space
Flight Center in Greenbelt, Md., said during a Google+ Hangout
announcing the findings. "But this is actually the first real
data that we have to give us the shape of the tail."

Though IBEX data has given scientists an idea of the shape and
structure of the heliotail, they say they have not been able to
measure its length particularly well. They think it is probably
evaporating over something like 1,000 times the distance between
the Earth and the sun, McComas said.

The lobes of the clover shape do not line up with the solar
system perfectly, the scientists found. A slightly rotated shape
hints that the particles in the tail are tugged by the magnetic
fields from the local galaxy as they move further away from the
sun.

Continued observations, hopefully with IBEX, should be able to
track how the tail shape shifts during the sun's 11-year activity
cycle, which is reaching its maximum this year, McComas
said.

"We should be able to see the effects of the changing solar
wind," he added during today's briefing.

The $169 million IBEX spacecraft, launched in 2008, was built for
an initial two-year mission, which has since been extended.
Early on in its mission, IBEX detected ENAs flowing toward the
sun in an unexpected pattern: They were significantly enhanced in
a
mysterious ribbon on the edge of the solar system that
scientists now think is a reflection of the solar wind, shot back
toward the sun by a strong galactic magnetic field.

IBEX has made several other important discoveries throughout its
mission. In 2010, the spacecraft looked back toward Earth and got
the first-ever glimpse of the solar wind crashing into the
planet's magnetosphere. Last year, NASA announced that the
spacecraft made its first detection of matter from outside the
solar system,
finding alien particles of hydrogen, oxygen and neon in the
interstellar wind.